Main key telephone system casing



FIG. 1 is a front elevational view of a main key telephone system casing showing my new design;

FIG. 2 is a rear elevational view;

FIG. 3 is a left side elevational view;

FIG. 4 is a right side elevational view;

FIG. 5 is a top plan view;

FIG. 6 is a bottom plan view; and

FIG. 7 is a left side and front perspective view thereof. 

The ornamental design for a main key telephone system casing, substantially as shown and described. 